In general, in order to manufacture a desired semiconductor integrated circuit, various thermal processes including a film-forming process, an etching process, an oxidation process, a diffusion process, a modifying process or the like are carried out to a semiconductor wafer, which consists of a silicon substrate or the like. These thermal processes may be conducted by a longitudinal batch-type of thermal processing unit. In the case, at first, from a cassette that can contain a plurality of, for example 25 semiconductor wafers, semiconductor wafers are conveyed onto a longitudinal wafer boat. For example, 30 to 150 wafers (depending on the wafer size) are placed on the wafer boat in a tier-like manner. The wafer boat is conveyed (loaded) into a processing container that can be exhausted, through a lower portion thereof. After that, the inside of the processing container is maintained at an airtight state. Then, various process conditions including a flow rate of a process gas, a process pressure, a process temperature or the like are controlled to conduct a predetermined thermal process.
Herein, with reference to FIG. 10, an example of a conventional thermal processing unit is explained. The thermal processing unit 2 has a longitudinal processing container 8 with a predetermined length, which has a double-tube structure of an inner tube 4 and an outer tube 6 and is made of quartz. In a processing space S in the inner tube 4, a wafer boat 10 made of quartz is contained as a holder for holding the object to be processed. Semiconductor wafers W as objects to be processed are held on the wafer boat 10 in a tier-like manner at a predetermined interval (pitch).
A cap 12 is provided for opening and closing a lower portion of the processing container 8. A rotation shaft 16, which can rotate via a magnetic-fluid seal 14, is provided at the cap 12. A rotation table 18 is provided on an upper end of the rotation shaft 16. A heat-insulating cylinder 20 is provided on the table 18. The wafer boat 10 is placed on the heat-insulating cylinder 20. The cap 12 is attached to an arm 24 of a boat elevator 22 that can be moved up and down, so that the cap 12 can be moved up and down together with the rotation shaft 16 and the wafer boat 10 and the like. Because of the up-and-down movement by the boat elevator 22, the wafer boat 10 can be inserted into or taken out from the processing container 8 through a bottom portion of the processing container 8.
A manifold 26 made of for example stainless steel is joined to a lower-end opening of the processing container 8. A plurality of (two in the shown example) gas nozzles 28A, 28B penetrates the manifold 26 for introducing various process gases necessary for a thermal process, for example a film-forming process, into the processing container 8. The respective gas nozzles 28A, 28B are connected to respective gas supplying systems 30A, 30B. In the respective gas supplying systems 30A, 30B, flow-rate controllers 32A, 32B such as mass-flow controllers that can control gas flow rates are respectively provided.
Then, the respective process gases supplied from the respective gas nozzles 28A, 28B ascend in the processing space S (wafer containing region) in the inner tube 4, turn down at a ceiling portion and descend in a gap between the inner tube 4 and the outer tube 6.
A gas-discharging port 34 communicating with the gap between the inner tube 4 and the outer tube 6 is provided at a side wall of the manifold 26. A vacuum pump not shown and the like are connected to the gas-discharging port 34. Thus, a vacuum can be created in the processing container 8.
A heat-insulating barrier 36 consisting of a heat-insulating material is provided outside the processing container 8. A heater 38 as a heating means is provided inside the heat-insulating barrier 36. Thus, the wafers W in the processing container 8 are adapted to be heated to a predetermined temperature.
The conventional thermal processing unit 2 is designed to conduct a thermal process, such as a film-forming process or an oxidation-diffusion process, at a relatively high temperature zone of for example 900 to 1200° C. In view of thermal stability at the relatively high temperature zone and so on, the heat-insulating barrier 36 is designed to be relatively thick so as to have large thermal capacity. In addition, in order to rapidly lower the temperature of the processed wafers, a thermal processing unit has been proposed wherein a cool wind is blown to an outside surface of the processing container (for example, JP laid-Open Publication No. 2000-100812).
Recently, it is possible that the semiconductor wafers have to be thermally processed at a relatively low temperature zone of for example 50 to 600° C., instead of the relatively high temperature zone of for example 900 to 1200° C. For example, in order to grant a request of high-speed operation of a semiconductor device, if copper wiring that has been recently paid attention to is formed to reduce wiring resistance, it may be necessary to conduct an annealing process to copper films coated on the wafers at a relatively low temperature of about 50 to 150° C. In addition, in order to reduce wiring capacity, if an organic film whose dielectric constant is small, such as a resin, is used as an interlayer dielectric film, it may be necessary to vitrify the organic film at a relatively low temperature of about 400 to 600° C.
When a thermal process is conducted at the above low temperature zone, if the thermal processing unit 2 as shown in FIG. 6, which is designed for the high temperature zone of for example 900 to 1200° C. and which has the large thermal capacity, is used, it may take a very long time to lower the wafer temperature to a handling temperature that is around a room temperature, even if the thermal process is conducted at the low temperature. For example, as described above, since the heat-insulating barrier 36 is thick and has the large thermal capacity, a rate of temperature fall in the high-temperature zone of about 900 to 1200° C. is 5 to 6° C./min, which is large, but a rate of temperature fall in the low temperature zone of around 100° C. is 1 to 2° C./min, which is very small. The phenomenon in the low temperature zone may be also found in another unit wherein a cooling wind is blown to a side wall of a processing container 8.
If it takes a long time to lower the temperature of the processed wafers to the handling temperature, throughput is remarkably deteriorated.